# LeetCode Implement Queue using Stacks

LeetCode Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only `push to top`, `peek/pop from top`, `size`, and `is empty` operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

```class MyQueue {
private:
stack<int> stk1, stk2; // 输入来源1,2,3，stk1从栈底到栈顶的顺序是1,2,3，stk2从栈底到栈顶的顺序是3,2,1
public:
/** Initialize your data structure here. */
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
while (!stk2.empty()) {
stk1.push(stk2.top());
stk2.pop();
}
stk1.push(x);
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
while (!stk1.empty()) {
stk2.push(stk1.top());
stk1.pop();
}
int top = stk2.top();
stk2.pop();
}

/** Get the front element. */
int peek() {
while (!stk1.empty()) {
stk2.push(stk1.top());
stk1.pop();
}
return stk2.top();
}

/** Returns whether the queue is empty. */
bool empty() {
return stk1.empty() && stk2.empty();
}
};
```

```class MyQueue {
private:
stack<int> stk1, stk2;
while(!stk1.empty()) {
stk2.push(stk1.top());
stk1.pop();
}
}
public:
/** Initialize your data structure here. */
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
stk1.push(x);
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
int ans = stk2.top();
stk2.pop();
return ans;
}

/** Get the front element. */
int peek() {
int ans = stk2.top();
return ans;
}

/** Returns whether the queue is empty. */
bool empty() {
return stk1.empty() && stk2.empty();
}
};
```